Method of making spark plugs



- Feb. 23, 1943. c. A. KUS HLER METHOD OF MAKING SPARK PLUGS Filed Oct. 26, 1940 I all l i I I I sufllcient to prevent flash-over.

Patented Feb. 23, 1943 METHOD OF'MAKING SPARK PLUGS Clarence A. Kushler, Detroit, Mich, assignor to Champion Spark Plug Company, Toledo, Ohio, a corporation of Delaware Application October 26, 1940, Serial No. 362,922

2 Claims. (Cl. 29-15551) This invention relates to spark plugs and has for its primary object to reduce the likelihood of flash-over between the oppositely charged metallic portions of a spark plug.

When a spark plug is operating in an internal combustion engine of the modern high compression type there exists a diiference of potential of several thousand volts between the exposed terminal cap which protrudes from the insulating portion of the plug and the shell or cylinder engaging surface. Not only is the difference of potentialgreat, but the frequency of voltage fluctuation from zero to the maximum is very high. Under normal, dry conditions and at normal atmospheric pressures, the air gap between the ter minal and shell of the spark plug is more than However, as the plug surface becomes fouled and picks up moisture or if the plugs are operated in rarified atmos pheres, the likelihood of failure by reason .of flash-over across the exterior of the plug is increased.

The flash-over problem has been particularly noticeable in connection with so-called onepiece plugs which are assembled by rolling the upper end portion of the shell in over a mass of insulating material. To facilitate the rolling operation the end of the shell is usually tapered or made narrow in cross-section and while the assembling machines are designed to force the end of the shell as close to the core or insulating body as possible, there very frequently exists a decided gap between the core and the turned in portion of the shell. It is impractical to operate assembling machines in such a way as to completely elimihate-this air-gap because a slight increase in the initial length of the shell might lead to an operation in which the machines would cause a positive and forceful abutment of the end of the shell against the core sufllcient to produce a crack in the latter which might possibly go unnoticed in subsequent inspections.

1" The presence of the air gap between the core and overturned shell part means that the dielectric between the center electrode and the shell consists of two substances having widely varying dielectric constants. In the ordinary ceramic cored spark plug the ceramic has a dielectric constant of approximately 5, while the air has a dielectric constant of 7.. The potential gradient across the two dielectrics is inversely proportional to their dielectric constants. This fact leads frequently to a breakdown of the air gap by ioniza tion of the air and establishment of a corona. discharge at-that point. -While under most circum- CIT stances the ionized air of the corona is dispersed before any opportunity for growth or further breakdown is had, the presence of this ionized body has frequently led to failure of the spark plug by flash-over since it materially reduces the leakage path between the terminal and shell parts of the plug over which the flash-over current must flow.

The present invention completely obviates this difilculty as will be apparent from the following specification taken in connection with the accompanying drawing which shows'a spark plug embodying the invention half in elevation and half in central vertical section.

Referring to the drawing, the spark plug con sists of a core I of insulating material which is seated in a shell 2 and held therein in any suitable manner. In the form shown the plug is a one piece structure and the core is held in by a mass of insulating material 4 interposed between a shoulder 5 of the core and over which is disposed an inturned shell part 3. If desired, the inter mediate insulation 4 can be eliminated and the inturned shell part made to lie directly against the shoulder of the core.

A center electrode 6 is provided to carry cur rent from the end terminal I to the gap. The ap, of course, is made up of the inner end of the center electrode 5 and a suitable shell electrode 1 which is maintained at ground potential. The center electrode is fixed in, the core by cement or by any other known means.

In assembling a one-piece spark plug, as here tofore mentioned, the portion 3 is forced inwardly and lies very close to the adjacent surface of the core E. However, a small air gap G which is exaggerated in the present drawing is established by reason of the inability of the assembling machines to make a perfect abutting contact between the two parts. It is this air gap and the character of the metal edge which forms it which leads to the disruptive effects heretofore explained As prevlously mentioned, the upper end of the shell is tapered to facilitate rolling it in. leads to a concentrated electrostatic field, and if the edge is uneven or ragged as it is apt to be after assembly, the field is further concentrated at the sharp points.

The present invention is, as above stated, partlcularly directed to a method and means for preventing flash-over between the terminal I and the inturned portion 3 of the shell arid consist primarily in the complete elimination of the air gap G. For this purpose a coating of dense, homogeneous material In is applied-to th shell and is permitted to flow into'and fill the air gap andto extend slightly up the surface of the core, as indicated at H. Preferably, the material I0 is such that its dielectric constant is of the same order as.

the dielectric constant of the core I, although this is not to be construed as a limiting factor.

In the preferred form, insulating material is used for the coating I 0, and it is preferable to extend it down over the surface of the shell for a short distance in order to lengthen the leakage path between the terminal I and the closest exposed A portion of the shell. It will be appreciated, however, that the coating I0 may be made of metal. since the air gap will be satisfactorily eliminated and there is sufficient dielectric strength in the core itself to prevent breakdown, it being only necessary to crowd the metal between the core and shell to eliminate the overstressed air gap with its resulting corona discharge. In case metal' is used, when applied, it should be less rigid and more workable than the rolled-over portion 3 of the shell.

While the invention has been particularly described in connection with "one-piece plugs, it

willbe appreciated that it is applicable to any spark plug in which the insulation between the center electro and the shell is'made up of two substances having widely varied dielectric constants, such, for example, as ceramic material and air so disposed as to result in exposed corona upon ionization of the air.

What I claim is:

1. The method of making a spark plug which 2. The method of making a spark plug which 4 includes assembling a core and shell, rolling in a portion of the shell to overlie a portion of the core with the end of the rolled in portion spaced slightly from the adjacent surface of the core, and subsequently applying a material more plastic than the shell and having dielectric properties 1 similar to the core to completely fill the space between the core and shell and to extend slightly over the adjacent surface of both the core and shell.

' CLARENCE KUSHLER. 

